Skin epidermis consists of four to five stratified layers, all of which comprise mostly keratinocytes; other cell types such as fibroblasts also populate the epidermis. Keratinocytes originate from the bottom-most, basal layer of the epidermis, and gradually migrate to the most exterior portion of the skin, where they become cornified and eventually slough. During this migration, keratinocytes differentiate to express the enzymes and structural proteins necessary for cornification (Presland and Dale, 2000). Given their prominent role in forming the epidermis, keratinocytes represent a main target for treating damaged skin.
Keratinocytes are also a main constituent of mucosal tissues that are continuous with the epidermis (Presland and Dale, 2000). Such tissue lacks the impermeable, cornified layer of the epidermis, and forms the inner-lining surfaces associated with the mouth, nose, throat, ear, anus and genitalia. Similar to the skin, mucosal surfaces are important for preventing entry of infectious agents into the body; thus, injury to either of these tissue types may compromise the health of an individual.
Skin and mucosal tissue damage occurs when the epidermal layer is breached, such as from a laceration, burn or blister. Injury can also involve crushing or bruising, which involves tissue damage without concurrent fissure of the epidermis. Skin infections as well as certain chronic illnesses such as cancer and autoimmune diseases can also exact a toll on epidermal surfaces. Ulcers, such as those affecting diabetics or those associated with pressure sores, are another form of skin damage; these wounds are often quite intractable, being inflamed, prone to infection, and requiring a lengthy healing process. It is generally posited that the persistence of an ulcer, or any other chronic wound, is due to a failure of cellular processes involved in healing, such as cell signaling (Enoch and Price, 2004; Sweitzer et al., 2006). One failure is the inability to epithelialize the lesion—keratinocytes at the wound border, though able to proliferate, do not mobilize to cover the sore (Enoch and Price, 2004). In relation to diabetic ulcers, another failure is the lack of certain signaling molecules; this deficiency which may preclude the remodeling processes that are necessary for orchestrating wound closure (Sweitzer et al., 2006).
Other forms of epidermal damage are subtle and result over a long period of time, eventually compromising skin function in the face of acute injury; wound healing is extended in time and can be imperfect (e.g. scar formation). Cosmetic problems such as wrinkling, dryness, thinning, sagging and greater susceptibility to bruising are usual outward signs of such maladies. Not surprisingly, these signs of wear-and-tear are usually associated with aging, but can also occur prematurely due to prolonged exposure to damaging agents such as ultraviolet rays (photoaging). The photoreactive processes induced by sunlight can contribute to reduced skin thickness and elasticity, as well as increased skin toughening (Pelicci, 2004; Fisher et al., 2002).
Healing of acute skin and mucosal wounds is orchestrated, in part, through the activation of basal keratinocytes, which follow a path of proliferation, migration and differentiation to effect wound closure. This process is accompanied by an array of remodeling activities at the injury site (Enoch and Price, 2004). Keratinocytes located at the wound perimeter proliferate and migrate to form a single layer over the wound in a process referred to as epithelialization. Further proliferation and differentiation of the keratinocytes establishes an epidermal layer comprising the normal stratified layers. Inflammatory processes may facilitate wound healing; infiltrating monocytes fight infection and also release factors that stimulate wound epithelialization. However, inflammatory processes can also aggravate healing; for example, fibrin deposition by macrophages contributes to scarring. So long as antiseptic conditions are maintained, it has been shown that epidermal wound closure occurs faster and with less scarring when immune involvement is curtailed (Martin and Leibovich, 2005). It is with these insights in mind that modes for down-modulating inflammation at epidermal lesions are currently contemplated.
Several factors have been shown to spur epithelialization by keratinocytes during wound healing in skin and associated mucosal tissues, including epidermal growth factor (EGF), basic fibroblast growth factor (bFGF), keratinocyte growth factor (KGF), and platelet-derived growth factors (PDGFs) (Enoch and Price, 2004). Interestingly, several antimicrobial proteins that are present on skin and mucosal surfaces are also known to play a role in stimulating the cell proliferation and migration required for healing epidermal wounds (Shaykhiev et al., 2005; Braff and Gallo, 2006; Zhang and Falla, 2006).
With the knowledge that certain growth factors are naturally engaged during wound healing, work has been directed towards developing growth factor-based methods for treating wounds, especially those that are generally chronic. For example, treatment of diabetic ulcers with PDGF-BB (Mustoe et al., 1994; Steed, 1995) has gained FDA approval. However, most attempts employing such a strategy have failed to achieve clinically significant results, due in part to difficulties associated with use of therapeutic proteins. One problem relates to the inefficient delivery of growth factors to wound sites; topical application of these proteins only permits exposure of exterior, mostly dead tissue to the therapeutic protein. Other drawbacks relate to the high lability and poor retention of growth factor proteins after delivery to the wound site.
Difficulties with the use of growth factors and other proteins to treat epidermal wounds are related to the large size of the proteins involved. Widespread use of growth factor therapies also suffers from the complexity and high costs associated with preparing large proteins. Therefore, as it concerns the use of protein factors in wound healing regimens, less expensive and more effective preparations are presently sought. Short peptides that bear the activity of the larger proteins from which they are derived (i.e. parent protein) fill this need. Previous examples of such short peptides have been reported (U.S. Pat. Nos. 6,861,406 and 6,693,077; Lee et al., 2004). Besides the immediate benefits of less expensive, more simple production, handling, and manipulation, small bio-active peptides are also better absorbed and retained by wound tissue. The advanced absorption characteristics of short bio-active peptides also make them a viable option for uses beyond the care of acute and chronic lesions, such as for treatment of the skin problems associated with old age and sun exposure.